By Adam Varenhorst
Most of South Dakota was dry during 2021, which has set up the possibility for larger than normal grasshopper populations for this season. We have already observed increased grasshopper activity in many areas of the state and, depending on the 2022 season, they may become problematic in crops.
Grasshopper activity in pastures and field edges can cause reduction in forage (Figure 1), and activity in crops can lead to reduced stands or yield loss.
There are three main species of grasshoppers that we monitor for in South Dakota: two-striped, redlegged and differential grasshoppers.
Figure 1. An alfalfa/hay mix pasture that has been impacted by grasshopper feeding.
Identification
TWO-STRIPED GRASSHOPPER
Figure 2. Two-striped grasshopper adult.
Two-striped grasshoppers get their name because of the two light-yellow lines that run on the back of their head to their abdomen, where they converge near the middle of the wings (Figure 2). Two-striped grasshoppers are normally tan to brown in color. They are approximately 1 to 1 ½ inches long.
REDLEGGED GRASSHOPPER
Figure 3. Redlegged grasshopper adult.
Another grasshopper that can be observed is the redlegged grasshopper. These grasshoppers get their name from their red hind leg segment (Figure 3). Their bodies range in color from green to tan with black markings, and they are 2/3 to 1 inch long.
DIFFERENTIAL GRASSHOPPER
Figure 4. Differential grasshopper adult.
The other large grasshopper species that can be observed are the differential grasshoppers. These vary in size from 1 1/8 to 1 ½ inches long. Their bodies are green to yellow in color, and they have black chevron markings on their hind legs (Figure 4).
Scouting and Management Options
The two methods for determining if populations are at or above threshold are to use visual counts or a sweep net. For visual counts, estimate a square yard in front of you and count the number of grasshoppers jumping out of the area as you slowly walk towards it. For sweep netting, use a 15-inch diameter sweep net and capture grasshoppers in four pendulum swings (approximately one square yard of area) and count them. With either sampling method, we recommend repeating it several times in a pattern, such as the example in Figure 5, and calculating the average for increased accuracy.
The threshold for grasshopper populations that was established by the United States Department of Agriculture (USDA) for grasslands is 15 to 20 nymphs or 8 to 10 adults per square yard.
For crops, we recommend scouting for nymphs and adults, both in field margins and within the fields. For field margins, management should be considered when nymph populations reach 50 to 75 per square yard and adult populations reach 21 to 40 per square yard. In the field, management should be considered when there are 30 to 45 nymphs per square yard, or when there are 8 to 14 adults per square yard. If you observe silk feeding, or grasshoppers feeding on ear tips or developing kernels, insecticide management should be considered. For soybean, if 20% defoliation occurs after flowering or are feeding on developing pods, insecticide management should be considered.
If thresholds are exceeded, an insecticide that is labeled for grasshopper management may be considered. A current list of insecticide sprays for use in soybean, corn and alfalfa can be found in the latest South Dakota Pest Management Guides.
In pastures and range, spreading poison grasshopper baits or baits containing Nosema locusta spores may be used to reduce populations. If treating earlier in the season while grasshoppers are still growing, an insect growth regulator (IGR) containing diflubenzuron may be applied to inhibit grasshopper development. However, please note that IGRs have no effect on adult grasshoppers and should only be used to manage developing nymphs.
Source : sdstate.edu